The present invention relates generally to composite particles containing metallic shell layers. More, particularly the present invention relates to particles that include a shell layer that partially covers a substrate particle. The substrate particle may be a core particle or an inner composite particle. Further, the present invention relates to a method for making a composite particle that includes mechanically introducing asymmetry to the particle and forming a partial shell as determined by the asymmetry.
Particles able to absorb or scatter light of well-defined colors have been used in applications involving detection, absorption, or scattering of light, for example medical diagnostic imaging. Such particles are typically colloidal metal particles. The term colloidal conventionally refers to the size of the particles, generally denoting particles having a size between about 1 nanometer and about 1 micron.
Small particles made from certain metals that are in the size range of colloidal metal particles tend to have a particularly strong interaction with light, termed a resonance, with a maximum at a well-defined wavelength. Such metals include gold, silver, platinum, and, to a lesser extent, others of the transition metals. Light at the resonance wavelength excites particular collective modes of electrons, termed plasma modes, in the metal. Hence the resonance is termed the plasmon resonance.
By selecting the metal material of a colloidal particle, it possible to vary the wavelength of the plasmon resonance. When the plasmon resonance involves the absorption of light, this gives a solution of absorbing particles a well-defined color, since color depends on the wavelength of light that is absorbed. Solid gold colloidal particles have a characteristic absorption with a maximum at 500-530 nanometers, giving a solution of these particles a characteristic red color. The small variation in the wavelength results from a particle size dependence of the plasmon resonance. Alternatively, solid silver colloidal particles have a characteristic absorption with a maximum at 390-420 nanometers, giving a solution of these particles a characteristic yellow color.
Using small particles of various metals, particles can be made that exhibit absorption or scattering of selected characteristic colors across a visible spectrum. However, a solid metal colloidal particle absorbing in the infrared is not known. Optical extinction, in particular absorption or scattering, in the infrared is desirable for imaging methods that operate in the infrared. Further, optical communications, such as long distance phone service that is transmitted over optical fibers, operate in the infrared.
It has been speculated since the 1950""s that it would be theoretically possible to shift the plasmon resonance of a metal to longer wavelengths by forming a shell of that metal around a core particle made of a different material. In particular, the full calculation of scattering from a sphere of arbitrary material was solved by Mie, as described in G. Mie, Ann. Phys. 24, 377 (1908). This solution was extended to concentric spheres of different materials, using simplifying assumptions regarding the dielectric properties of the materials, by Aden and Kerker, as described in A. L. Aden and M. Kerker, J. of Applied Physics, 22, 10, 1242 (1951). The wavelength of the plasmon resonance would depend on the ratio of the thickness of the metal shell to the size, such as diameter of a sphere, of the core. In this manner, the plasmon resonance would be geometrically tunable, such as by varying the thickness of the shell layer. A disadvantage of this approach was its reliance on bulk dielectric properties of the materials. Thus, thin metal shells, with a thickness less than the mean free path of electrons in the shell, were not described.
Despite the theoretical speculation, early efforts to confirm tunability of the plasmon resonance were unsuccessful due to the inability to make a particle having a metal shell on a dielectric core with sufficient precision so as to have well-defined geometrical properties. In these earlier methods, it was difficult to achieve one or both of monodispersity of the dielectric core and a well-defined controllable thickness of a metal shell, both desirable properties for tuning the plasmon resonance. Thus, attempts to produce particles having a plasmon resonance in keeping with theoretical predictions tended to result instead in solutions of those particles having broad, ill-defined absorption spectra. In many instances this was because the methods of making the particles failed to produce smooth uniform metal shells. Rather, the methods tended to produce isotropic, non-uniform shells, for example shells having a bumpy surface.
However, one of the present inventors co-developed a novel method of making coated nanoparticles (particles with a size between about 1 nanometer and about 5 microns) that was successful in producing metal-coated particles having narrow well-defined spectra. Further, one of the present inventors co-discovered that improved agreement with theoretical modeling of the coated nanoparticles resulted from the incorporation in the theory of a non-bulk, size-dependent value of the electron mean free path. That is, improved agreement with theory was achieved by developing an improved theory applicable to thin metal coatings. Thus, in the improved theory a dependence of the width of the plasmon resonance on the thickness of the metal coating was described.
Complete nanoparticle coatings with gold have been demonstrated. Particles having at least one substantially uniform metal coating layer have been termed metal nanoshells. Nanoshell structures that exhibit structural tunability of optical resonance""s from the visible into the infrared can currently be fabricated. Gold has the advantage of a strong plasmon resonance that can be tuned by varying the thickness of the coating. More generally, the resonance may be tuned by varying either the core thickness or the thickness of the coating, in turn affecting the ratio of the thickness of the coating to the thickness of the core. This ratio determines the wavelength of the plasmon resonance. A further advantage of gold-coated particles is that they have shown promise as materials with advantages in imaging and diagnostics. In particular, they have utility as band-pass optical filters, impeding the photo-oxidation of conjugated polymers, and in conjunction with sensing devices based on surface enhanced Raman substrates.
However, plasmon resonant particles with enhanced nonlinear optical properties are desirable for applications in, for example, optical mixing and optical modulating. Nonliner optical effects are described as follows. When light of electric field E and oscillation frequency v is incident on a substance, a wave of polarization of the frequency omega. is induced in proportion to the electric field E in the substance. Then, light of the oscillation frequency v originates from the wave of polarization. This is normal interaction of light with a substance, and the incident light is identical in oscillation frequency with the outgoing light. In some particular substances, however, light of electric field E and oscillation frequency v induces considerably intense waves of polarization proportional to En. Substances of this nature are called nonlinear optical media. These substances show the following peculiar phenomena. They produce light having an oscillation frequency n times as high as the oscillation frequency of the incident light, i.e., the outgoing light shows a color different from that of the incident light. The refractive index of such a nonlinear optical medium may change as a function of the intensity of the light, or the square of the electric field. These are collectively known as nonlinear optical effects. Application of nonlinear optical effects to wavelength conversion of laser radiation and to optical logic devices is well known. One method of obtaining nonlinear optical effects is to introduce asymmetry. Thus, it would be desirable to provide anisiotropic plasmon resonant particles and a method for making them. However, a robust method of partially coating spherical nanoparticles with a controlled xe2x80x9csolid anglexe2x80x9d of coverage has thus far not been reported.
Further, it would be useful to have a method of making small metal-coated particles with other advantages, such magnetism arising from the metal coating. Small magnetic particles have many applications. Such articles are used as toner in xerography, in ferrofluid vacuum seals, in nuclear magnetic resonance imaging as contrast agents, and in magnetic data storage. These magnetic particles are typically micron-sized in diameter or larger. The large size of these particles renders them less than satisfactory for several specialized applications.
If the magnetic particles were smaller, cost reduction by reducing the number of processing steps would be achieved in xerographic applications. In ferrofluid applications, the enhanced solubility due to carbon coating provided by smaller particles may be advantageous. In magnetic data storage, high density may be enhanced by using smaller particles. Moreover, in magnetic ink applications, the carbon coating and ability to disperse the nanoparticles in aqueous solutions may provide advantages for wetting and coating. Further, there is a need for particles with enhanced magnetic properties, as may arise from asymmetry. Consequently, there is a potential need for anisotropic sub-micron-sized metal, alloy, or metal carbide particles with magnetic properties and a method for making them.
According to an embodiment, the present invention features an asymmetric nanoparticle that includes a substrate particle and a metallic shell layered on said substrate particle within a solid angle less that 360xc2x0. The substrate particle preferably includes a nonmetallic material. The nonmetallic material may be throughout the substrate particle. Alternatively, the nonmetallic material is contained in an outer layer of the substrate particle. The shell includes a metallic material. The metallic material may be selected from among elemental metals, alloys, and synthetic metals. The metallic material may be a plasmon resonant material. Alternatively, or in combination the metallic material may be a magnetic material.
According to an alternative embodiment, the present inventions features a method for making a composite particles that includes asymmetrically confining a substrate particle so as to define a first surface portion and a second surface portion and selectively layering a primary metallic material over either the first surface portion or the second surface portion so as to form a partial metal shell that includes the primary metallic material.
According to yet an alternative embodiment, the present invention features a method for making a composite particle that includes electrophoretically attaching a core particle to a metallized support such that the core particle has an exposed portion and a remaining portion, selectively attaching a plurality of activating agents to either the exposed portion or the remaining portion, depositing a plurality of metal colloids onto the activating agents, reducing additional metal onto the metal colloids so as to form a partial metal shell.
Thus, the present invention comprises a combination of features and advantages which enable it to overcome various problems of prior particles. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.